For certain applications, it is often desirable to irradiate a plastic material, such as with a high-energy electron beam, to cause material changes in the plastic. For example, it is known that the cross-linking of polymer plastic material is promoted through irradiation of the material by an electron beam. The electron beam must be of high enough energy and intensity so that a sufficient quantity of electrons penetrate to a sufficient depth into the material to provide the desired radiation dosage.
Providing a controlled radiation dosage radially into a strand or tube has resulted in advantageous material properties. A special case example requiring highly controlled radial beam dosage is the radially differentially cross-linked polymer tubing of the type described in commonly assigned U.S. Pat. No. 3,455,337. The differentially cross-linked polymer in tubular form, such as for heat shrink tubing applications, includes a fusible noncross-linked inner concentric portion that may be used to seal an underlying electrical connection, while the outer cross-linked concentric portion acts as a conventional heat shrink tubing.
Providing a circumferentially uniform dosage of radiation to a strand is relatively difficult to achieve. In one approach, an electron beam of sufficient energy has been passed through the entire cross-sectional dimension of material which was advanced in a single pass through the electron beam. Unfortunately, for a typical application for irradiating a plastic strand to manufacture heat shrink tubing, the required radiation source must be capable of generating an electron beam of a million electron volts or more. Accordingly, such an apparatus is very large, and must be contained within an extensively shielded enclosure to protect against potentially hazardous radiation exposure.
Another single pass approach to irradiating a strand, known as the "torroid" approach, used a torroidal vacuum chamber and circular electron gun to emit a beam around 360.degree.. This approach, however, has not proven to be commercially practical or successful. Another variation similar to the torroid approach is to use three electron guns disposed about a coaxial window with 120.degree. displacement between adjacent guns. This approach has also proven to be commercially impractical.
U.S. Pat. No. 5,051,600, assigned to the assignee of the present invention, discloses an apparatus for more uniformly irradiating an advancing strand in a single pass. The apparatus includes two opposing electron beam generators and respective deflector magnets and convergence magnets for causing relatively uniform radiation of the strand.
Another prior approach for irradiating plastic strands involves multiple passes of the strand in the path of the electron beam. The strand is manually threaded from a feed reel about sets of idler rolls in multiple loops and thus exposed in multiple loops to a scanned high energy electron beam. The amount and uniformity of irradiation has depended upon the scan rate of the beam and the advancement rate of the strand. The degree of uniformity of this approach is hampered because the strand often takes a particular set (reel set) when wound on a storage reel after extrusion. This reel set is maintained in the strand during the irradiation process thereby producing poor uniformity. Some improvement in uniformity for the multiple pass approach has been achieved by providing a magnet beneath the strand loops to reverse the direction of some portion of the electrons and theoretically return them to the sides of the loops for greater uniformity.
Despite numerous attempts to obtain a circumferentially uniformly irradiated strand, there still exists a need for an apparatus and method which overcome the limitations and drawbacks of the prior art.